Calculating milligrams (mg) per millilitre (ml) is a common task when you work with chemicals of all kinds. It is especially important when determining drug dosages, since the active ingredient in most drugs that are in liquid form is usually measured in mg per ml, or a percentage equivalent. A unit weight of 1 mg equals 0.001g. A unit volume of 1ml equals 0.001 l and is equivalent to 1 cubic cm.

Look up the density of a material in a table of densities (see References) if you want to determine how many milligrams are in 1ml of the material. Density is usually expressed as a proportion of the weight of an equal volume of water. For example, the density of gasoline relative to water is 0.70.

Multiply the density of the material by 1,000 to find the milligrams per millilitre. A millilitre of water contains 1,000 mg. For the gasoline example, multiply 0.70 times 1,000. Thus, there are 700 mg of gasoline in 1ml of gasoline.

Convert percentages of a drug to milligrams per millilitre. Most liquid drugs contain the active medication dissolved in water. The amount of the medication is often expressed as a percentage of the weight of water. For instance, a drug might be listed as a 2.5 per cent solution, which means 2.5 per cent of the weight of the liquid is active medicine.

Multiply the percentage by weight by 1,000 to find the milligrams per millilitre. For a 2.5 per cent solution, you would multiply 0.025 by 1,000, which equals 25ml. So, there are 25 mg of the drug in 1ml of the solution.

#### Tip

Once you know the milligrams per milliter of a drug, itâ€™s easy to figure dosage by volume. For instance, if you need a dosage of 60ml of a drug in a 2.5 per cent solution, that works out to 25 mg per milliter. Divide the dosage needed by the milligrams per millilitre to find the number of millilitres you need for the dose. Here, you have 60/25, or 2.4ml.